It can mean anything, really. Syntax errors can confuse you.
The best thing to do is look for undeclared identifiers (compiler doesn't always complain about them) and checking for mistakes, such as syntax errors on previous lines. Check for missing ; and such.

Originally Posted by Adak

io.h certainly IS included in some modern compilers. It is no longer part of the standard for C, but it is nevertheless, included in the very latest Pelles C versions.

Originally Posted by Salem

You mean it's included as a crutch to help ancient programmers limp along without them having to relearn too much.